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  3. Some desert regions get no rainfall for months, and even years. Yet water can be found if you dig deep enough. For a long time t

Some desert regions get no rainfall for months, and even years. Yet water can be found if you dig deep enough. For a long time t

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问题     Some desert regions get no rainfall for months, and even years. Yet water can be found if you dig deep enough. For a long time this was a puzzling question. Then geologists found the answer. The underground water is rainwater--but the rain fell hundreds of miles away! It soaked into the ground and then flowed underground through a rock sandwich.
    A rock sandwich with water in the filling has a scientific name: aquifer. An aquifer is composed of two or more layers of nonporous rock. That’s the bread of the sandwich. The filling is a layer of porous rock or sand. The whole sandwich, or aquifer, is titled at a slant. The higher end is in a region of good rainfall, where the rain soaks into the porous rock or sand. The rainwater flows down at a slant between the nonporous rock layers. If a well is drilled through the top layer, the water flows up into the well.
    If the upper end of the aquifer is higher than the top of the well, the water is forced up without pumping. This convenient arrangement is called an artesian well because this kind of well was first developed in Artois, France.
This passage is about ______.
选项 A、freshwater seas
B、salt water
C、mountain streams
D、underground water
答案D
解析 全文讲的是地下水的来龙去脉。讲了有些沙漠地区的水的来源，水流向该沙漠地区的渠道，以及这些沙漠地区地下水的取用。选项D是正确的。
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